As shown in the diagram of FIG. 1, certain undercarriages are fitted with a steering member 10, here actuators in a push-pull configuration, serving to swivel a steerable bottom portion 11 carrying the wheels 12 of the undercarriage 9. The angular position of the steerable bottom portion is controlled on the basis of a setpoint generated by a steering computer 1, or by the pilot acting on pedals 2 or on a steering wheel 3.
This angular position setpoint is used in a servo-control system (e.g. of the proportional integral derivative (PID)) type that compares the angular position setpoint with a measured angular position of the steerable bottom portion, and in the event of a difference, the steering member is activated to cause the steerable bottom portion to swivel until the measured angular position is equal to the setpoint angular position. For this purpose, the aircraft has a hydraulic unit 4 that receives fluid under pressure and that is fitted with a servo-valve 5 for delivering the fluid in response to an order 6 coming from the steering computer 1, which fluid is delivered to the actuators of the steering member 10. In general, activation of steering control is authorized only if the aircraft is on the ground, with the undercarriage 9 then being compressed at least in part against an internal suspension force generated by the shock absorber internal to the undercarriage 9. For this purpose, an isolation valve 7 is controlled by the steering computer 1 to admit fluid under pressure into the hydraulic block 4 in response to detecting that the aircraft is on the ground.
Measuring the angular position of the steerable portion makes use of one or more sensors 8 arranged on the undercarriage in order to generate an electrical signal that is suitable for use in implementing such servo-control. In general, sensors are used that are arranged in such a manner that a departure in the signal from an electrical origin is proportional to a departure in the angular position from a particular angular position taken as an angular position origin, for example the sensors may be of the rotary variable differential transformer (RVDT) type in which the angle of rotation is proportional to the angle of rotation of the steerable bottom portion.
A procedure is generally provided for setting the electrical origin so that it matches the angular position origin. For this purpose, while the undercarriage is compressed at least in part, the aircraft is caused to run on the ground so as to impart an angular position on the steerable bottom portion that leads to the aircraft running in a straight line. This angular position is taken as the angular position origin, and the electrical origin of the angular position measurement signal is caused to correspond thereto.
Nevertheless, the angular position origin and the electrical origin can drift over time for various reasons (wear, maladjustment, . . . ), such that the two origins no longer correspond. Under such circumstances, the pilot is obliged to act continuously to request a small angular deviation in order to force the aircraft to run in a straight line.